1. Field of the Invention
This invention relates to rechargeable metal oxide/lanthanum nickel hydride storage batteries.
2. Prior Art
The prior art is replete with patents dealing with systems which store hydrogen as a reduced compound rather than a gas at higher pressures within the cell chamber. The use of hexagonal intermetallic compounds of the generalized composition AB.sub.5, where A represents a rare-earth metal and B represents nickel or cobalt, have been suggested since they are known to easily absorb and desorb large quantities of hydrogen gas under relatively small pressures at ambient conditions. The rare-earth metal has generally been lanthanum, although other rare-earth metal compounds may be used for hydrogen storage. Prior art secondary batteries utilizing lanthanum nickel are shown in U.S. Pat. Nos. 3,850,694, Dunlop, et al.; 3,874,928, Will; and 3,959,018, Dunlop, et al. The hallmark of prior art batteries as typified by the latter two patents is the use of a solid positive electrode, and a spaced hydrogen gas diffusion negative electrode of lanthanum nickel spaced from the cathode having various degrees of immersion in an alkaline electrolyte. In the '694 patent, the variation is shown wherein the hydride is stored on the walls of the pressure container and not a part of the assembled electrode stack.
In copending U.S. application Ser. No. 782,158, a basic modification is shown wherein the lanthanum nickel forms a part of the negative electrode by the use of an unalloyed composition of LaNi.sub.5. Accordingly, in these systems, lanthanum nickel, as the hydrogen storing agent, is shown to be a part of the substantive electrode stack.
Several problems have occurred with these prior art constructions. In the case of the metal oxide/LaNi.sub.5 Hx cell according to the construction shown in Will ('928), the metal oxide and hydride electrodes are completely immersed in the alkaline electrolyte, generally KOH. The hermetically sealed cell of that construction has exhibited problems due to oxygen and hydrogen build-up during overcharge, as well as the problem of contamination of the hydride electrode by oxygen. The first problem, oxygen and hydrogen build-up, is not dealt with adequately in the prior art since it is assumed that the hydride will store excess hydrogen during overcharge. The essential problem of avoiding excess pressure build-up within the cell is one requiring matching the state of the charge of both electrodes so that they reach full charge at the same time. Hence, all the gas produced will be recombined as charging proceeds. The prior art does not account for the problem and, accordingly, consideration of potential solutions is not found.
The problem of contamination is one where the hydride electrode is corroded by the presence of oxygen thereby reducing its capacity. Clearly, the problem of contamination of the hydride electrode contributes to the corollary problem of hydrogen build-up since the decrease in efficiency of the hydride electrode carries with it the commensurate decrease in storage capacity of hydrogen.
A second type of construction utilizes an electrode stack similar to that found in NiCd cells. A porous separator, partially wetted with electrolyte, is placed between adjacent metal oxide and hydride electrodes. This is shown in the copending U.S. application Ser. No. 782,185. Oxygen produced during overcharge at the metal oxide electrode will diffuse through the separator to recombine with hydrogen at the hydride electrode. While the problem of oxygen build-up is avoided, this solution compromises the integrity of the cell by considerable corrosion of the hydride material itself.
Accordingly, the problem of recombination of oxygen, during overcharge, remains a significant area of research in metal oxide/rare-earth hydride secondary batteries. Within the prior art, the use of reducing electrodes, alternatively known as auxiliary or oxygen-consuming electrodes, have been established in other types of batteries, primarily nickel cadmium batteries. Oxygen build-up within nickel cadmium batteries is reduced by the use of an oxygen-consuming electrode used to form a hydroxide ion. References such as Seiger, U.S. Pat. No. 3,350,225, show the use of a consuming electrode in the context of nickel cadmium battery technology. Hence, the prior art Seiger patent relates to a rechargeable field dry cell secondary battery. The plates therein undergo reversible chemical reactions of oxidation and combination during charge and discharge, and the electrolyte is impregnated in the separator between the plates. Seiger specifically utilizes a porous oxygen-consuming electrode within the casing such that, during overcharge, the additional water which is generated as a consequence of the chemical reaction spreads over the surface area of the oxygen-consuming electrode. By means of terminals connecting the oxygen-consuming electrode to the negative terminal of the battery, the oxygen generated at each positive plate will migrate to the area of the oxygen-consuming electrode where it is then consumed at the surface at a rate preventing excessive pressure build-up within the sealed casing.
In the nickel cadmium battery technology, the reduction of oxygen build-up has been addressed as typified by Seiger. However, complete scavenging of oxygen gas is an unnecessary consideration since the cadmium electrode is not damaged by oxygen as opposed to a hydride electrode. Furthermore, the simultaneous avoidance of oxygen and hydrogen build-up is unnecessary in this system. Accordingly, in the particular technology of Seiger, the use of a scavenging electrode for purposes of reducing generated oxygen to water provides a common solution to the area of unwanted oxygen build-up. In terms of avoidance of both oxygen and hydrogen pressures, the prior art is totally devoid of any considerations in the context of the metal oxide/rare-earth nickel hydride secondary battery. The problem appears to be addressed in the context of a different battery disclosed in Yehiely, U.S. Pat. No. 3,470,025. In the context of an entirely different type of system utilizing alkaline battery cells, a control cell is provided in the Yehiely patent responsive to voltage rises above threshold levels representing predetermined discharge conditions. The control cell is utilized for reducing the discharge current from the serially connected cells to a safe level thereby suppressing the tendency towards damage. However, Yehiely is not concerned with the particular battery technology utilized herein.